Hermetic compressors of refrigeration present, mounted inside a hermetically sealed shell, a cylinder block sustaining a vertical crankshaft, to which is mounted the rotor of an electric motor. The weight of the crankshaft-rotor assembly is supported by an axial bearing generally in the form of a flat axial sliding bearing.
The crankshaft carries, at its lower end, a pump rotor that, during operation of the compressor, conducts lubricant oil from a reservoir defined in the lower portion of the shell to the parts with mutual relative movement, in order to guarantee oil supply for the adequate operation of said parts.
The position of the axial bearing may vary according to the arrangement of the compressor components and to design variations. The solutions consider mounting the rotor to the crankshaft below the cylinder block, such as illustrated in FIG. 1, or mounting the rotor to the crankshaft above the cylinder block, such as illustrated in FIG. 2. Depending on the mounting position of the rotor in relation to the cylinder block, the surfaces that define the axial bearing are altered.
In the situation in which the rotor is mounted below the cylinder block, the lower surface of an annular flange of the crankshaft is axially borne on an annular surface defined at the upper end of the radial bearing hub. On the other hand, when the rotor is mounted above the cylinder block, the lower face of the rotor is axially borne on an annular surface defined at the upper end of the radial bearing hub. However, when the rotor is mounted below the cylinder block, the lower surface of an annular flange of the crankshaft is axially borne on an annular surface defined at the upper end of the radial bearing hub.
In the compressors in which the rotor is mounted below the cylinder block, it is also known the arrangement in which a second bearing is provided radially actuating on the crankshaft, above the eccentric portion of the latter. In this construction, the crankshaft incorporates a second annular flange, whose lower face is axially borne on an upper annular surface of this second radial bearing.
In any of the above-mentioned embodiments, the perfect parallelism between the mutually confronting surfaces that define the axial bearing is not assured, due to the presence of position errors (axial strikes) and mainly to deformations of the components during the operation of the compressor.
The position errors of the surfaces that define the axial bearing can be minimized by using more precise manufacturing processes. However, the deformations of the components are inherent to the operation of the compressor and they are produced during the compression period of the refrigerant gases. These deformations are translated into loss of parallelism between the mutually confronting surfaces that define the axial bearing, resulting in a geometry that is unfavorable to the formation of an oil film, consequently reducing the capacity of sustaining the axial bearing, increasing the mechanical losses by friction and probably causing wear to the surfaces. In addition, the deformation of the components, more specifically the loss of perpendicularity that occurs between the connecting rod and the crankshaft, causes decomposition of the forces that compress the gases, giving origin to a component in the axial direction of the crankshaft, introducing an additional load to the force (weight) of the crankshaft-rotor assembly over the axial bearing.
The improvement in the energetic performance of these compressors can be obtained with the reduction of the mechanical friction losses, by using more efficient bearings. Within this concept, the use of an axial rolling bearing has been proposed, whose operation, in terms of dissipated mechanical loss, presents rates that are close to the ideal. A constructive solution of a bearing using this concept is described in the Brazilian patent PI 8503054 assigned to White Consolidated Industries, Inc. and regarding hermetic compressors in which the rotor of the electric motor is mounted above the cylinder block.
In this type of construction proposed in patent PI 8503054, the axial rolling bearing, which is composed by two annular flat races and by the ball cage, is provided between the rotor face and the annular surface defined at the upper end of the radial bearing hub, with the rolling bearing being guided, in the internal diameter thereof, directly by the external surface of the main body of the crankshaft.
The life of the axial rolling bearings is strongly influenced by the alignment of their races. Nevertheless, the existence of deviations, even of decimals of milliradians in the parallelism between the races, is sufficient to reduce their operational useful life in more than 20 times, as compared with the useful life of an axial rolling bearing with perfectly parallel races. This reduction in the useful life of the rolling bearings occurs due to the concentration of the axial load over one or two balls, instead of this load being distributed over all the balls of the rolling bearing.
In the hermetic compressors having the rotor of the electric motor mounted to the crankshaft below the cylinder block, the simple provision of an axial rolling bearing, such as suggested in patent PI 8503054, between the lower surface of an annular flange of the crankshaft and the annular surface defined at the upper end of the radial bearing hub, will increase the distance between the cylinder axis and said bearing annular surface that constitutes the adjacent end of the radial bearing block, as illustrated in FIG. 3. In this hypothetical mounting condition based on the prior art teachings considered herein, the increase of the distance between the cylinder axis and the adjacent end of the radial bearing hub will tend to cause a greater momentum on the radial bearing hub-crankshaft assembly, consequently increasing the bending and stresses that are applied to this assembly.
Another disadvantage of the embodiment illustrated in FIG. 3 refers to the high oil leakage that occurs throughout the axial rolling bearing, increasing the mechanical losses by viscous friction of the axial rolling bearing and reducing the amount of lubricant oil available in the crankshaft portion and in the components of the compressor mechanism located above the axial rolling bearing. The correct amount of lubricant oil available to the axial rolling bearing allows optimizing the mechanical losses and the useful life of this component.
The increase of the bending of the radial bearing hub-crankshaft assembly and the increase of the leakage throughout the axial rolling bearing increase the noise in the compressor, reduce the energetic efficiency of the bearings and reduce the mechanical reliability of the several compressor components, one of them being the axial rolling bearing.